public class Solution {
    public boolean isValidBST(TreeNode root) {
        return helper(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }

    private boolean helper(TreeNode root, long minValue, long maxValue) {
        if(root == null){
            return true;
        }
        if(root.val < minValue || root.val > maxValue){
            return false;
        }

        return helper(root.left, minValue, root.val) && helper(root.right, root.val, maxValue);
    }


}
class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode() {
    }

    TreeNode(int val) {
        this.val = val;
    }

    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}
